A readily available supply of plasma is an essential requirement of any medical trauma treatment facility. Since plasma can be stored at room temperature for only a matter of a few hours before spoilage occurs, it is conventional practice to freeze plasma. Typically, plasma is frozen within six hours after collection, in polyvinyl chloride bags holding about 250 milliliters. The fresh-frozen plasma is subsequently stored at temperatures of around --30.degree. C. When properly frozen, plasma may be stored for up to five years.
While the procedure of fresh-freezing plasma has essentially solved the problems of storing plasma, the process of thawing the frozen plasma for use presents certain difficulties. When other components of blood, such as whole blood or platelets, are being thawed, possible damage to cells during thawing is a major concern. For plasma, however, post-thaw viability of cellular structures is not of concern, but the viability of coagulation proteins is of primary importance. The most widely accepted method of thawing fresh-frozen plasma comprises immersing the bag of frozen plasma in a warm-water bath. By completely surrounding the bag of froze plasma in a 30.degree. C.-37.degree. C. water bath and agitating it periodically, a single bag or "unit" of frozen plasma may be thawed usually in thirty to forty-five minutes.
This procedure presents a number of problems. First, immersing the bag in a non-sterile water bath may contaminate the bag ports, such that the thawed plasma is tainted as it is withdrawn from the bag. Additionally, any interruption in the integrity of the bag can permit water to enter the bag, thereby contaminating the plasma. Further, the water bath process cannot be accelerated, such as by exposing the plasma to a higher temperature bath, since subjecting the frozen plasma to any larger thermal gradient in an effort to speed up the procedure can result in physical stress and possible damage to the normal protein configuration of the plasma. The requirement of a thawing period of from thirty to forty-five minutes renders the use of frozen plasma impractical for emergency trauma cases, where the patient may have an immediate need for plasma and cannot afford the luxury of waiting for frozen plasma to be thawed. As a result, it is a frequent practice for medical facilities to anticipate possible plasma needs by thawing a number of units of plasma in advance. However, since plasma cannot be safely refrozen once thawed, units which are thawed in anticipation of possible use must be discarded if the anticipated use does not arise.
Accordingly, there is a need to provide a hygienic method and apparatus for the thawing of fresh-frozen plasma which does not expose the ports or the contents of the bag to the possibility of contamination.
There is a further need to provide a method and apparatus for the thawing of fresh-frozen plasma which is sufficiently rapid that plasma can be kept frozen until only moments before it is actually needed.
A number of efforts have been made to adapt microwave ovens for thawing frozen blood components which are contained in a bag. Some of these efforts have involved attempts to adapt a conventional cavity-type microwave oven, of the type widely used for cooking foods, for use in thawing such blood components. However, it is well known that cavity-type microwave ovens suffer a number of disadvantages which, while merely an inconvenience in thawing frozen food, become of critical importance when thawing frozen components of blood for medical purposes. In a cavity resonator, microwaves bounce off the walls of the cavity. Superimposed incident and reflected waves can produce standing waves. At some locations within the cavity the incident and reflected electromagnetic waves reinforce one another, and at other locations the incident and reflected waves cancel. Thus, a typical resonant cavity may contain a number of electromagnetic "hot" spots where the incident and reflected waves reinforce one another, as well as a number of electromagnetic "cold" spots where the incident and reflected waves cancel one another. This uneven electromagnetic field pattern across the oven cavity will tend to heat some portions of a substance placed in its interior more rapidly than others, resulting in nonuniform thawing of the substance.
As if this were not problem enough, many substances, including plasma, exhibit widely varying absorption and dielectric characteristics as they change state from a solid to a liquid. Liquid plasma will absorb electromagnetic energy nearly seventeen times faster than frozen plasma. Thus, in an unevenly heated quantity of frozen plasma, as one particular portion of the material begins to thaw and changes to a liquid, it will begin absorbing electromagnetic energy considerably faster than its still-frozen surroundings. Further, as the amount of energy absorbed by the liquid plasma increases, the amount of remaining energy available to heat the adjacent frozen areas decreases. Thus, not only is the liquid plasma heated faster, but the adjacent areas of frozen plasma are heated even slower. This variation in absorption characteristics exacerbates the problem of uneven heating and results in a phenomenon known as "thermal runaway".
In addition, reflections of electromagnetic waves also occur at the interface between two insulating materials having differing dielectric or magnetic properties. Liquid plasma has a dielectric constant approximately ten times that of frozen plasma. Thus, as a quantity of frozen plasma is heated with microwaves, a dielectric discontinuity is created at the interface between liquid and solid plasma. An electromagnetic wave incident on such a boundary surface is partly transmitted into the second material and partly reflected back into the first. This reflection disrupts the electromagnetic field and can cause further uneven heating of the fresh-frozen plasma.
A number of efforts have been made to overcome these problems of nonuniform heating in a cavity-type resonator. In one method, a bag of plasma was exposed in a microwave oven for a number of thirty second periods. Between exposure periods, the bag was removed from the oven and manually manipulated for ten seconds to attempt to intermix any unevenly heated portions of the bag's contents. However, this method did not eliminate the problem of thermal runaway during the period of exposure to the microwave energy, and manual manipulation of the bags tended to produce inconsistent results.
In an effort to overcome these problems, an apparatus disclosed in U.S. Pat. No. 4,336,435 provides four rotary shafts in the oven cavity. A bag of plasma is clamped into a special bag holder, which in turn is snapped onto one of the rotary shafts. Each of the four rotary shafts is capable of accommodating a bag holder, permitting up to four bags to be thawed simultaneously. During thawing, the rotating shafts subject each bag to a rotary oscillating motion. While the plasma is solid, the agitation moves the plasma to avoid continuous exposure of any portion of the plasma to microwave "hot" or "cold" spots. As the plasma begins to thaw, the agitation purports to intermix the frozen and thawed portions of the plasma in an effort to attain temperature uniformity. A temperature probe associated with each rotary shaft monitors the temperature of each bag to avoid overheating. However, the apparatus imparts little motion to the bag adjacent to the axis of rotation of the rotary shift. Agitation of the plasma near the axis of rotation and movement of such plasma through microwave "hot" and "cold" spots is therefore not afforded. The apparatus is thus not completely effective in preventing overheating of localized portion of the plasma. Further, so that the bag of plasma will fit within the bag holder, the plasma must be frozen in a special clamping device to ensure an acceptable bag configuration, and bags frozen in an irregular shape cannot be accommodated by this apparatus.
In yet another attempt to adapt a cavity-type microwave oven for thawing plasma, U.S. Pat. No. 4,742,202 discloses a device mounted within the cavity of the oven which includes a tray for holding a bag of plasma. The tray travels around a three-dimensional circuitous track having peaks and valleys with respect to the oven floor. Simultaneously with travelling around this track, the tray may be rocked back and forth across a horizontal plane. Transporting the container of plasma around the track on the rocking ray avoids continuous exposure of the plasma to microwave "hot" and "cold" spots within the oven cavity. In addition, the transporting purports to agitate the plasma container sufficiently to cause a mixing between the warm and cold portions of the liquid. However, this apparatus has also not been completely effective in avoiding overheating of localized portions of the plasma. Further, this apparatus employs a bag holder similar to that disclosed in the aforementioned U.S. Pat. No. 4,336,435 and thus requires the plasma bags to be frozen in a configuration acceptable for accomodation by the bag holder.
A further complication inherent in using a cavity-type microwave oven concerns variations in the exposure time of a frozen substance depending upon the quantity of the substance involved. In a cavity-type resonator, electromagnetic waves will tend to continue bouncing around the interior of the cavity until they are absorbed by the material being thawed. If two bags of plasma are exposed within the oven at the same time, some of the microwave energy which would have otherwise reflected around inside the cavity until being absorbed by the first bag will instead be absorbed by the second bag and never reach the first bag. Thus, it takes longer for each bag to thaw, and exposure times for units of fresh-frozen plasma within a cavity-type microwave oven must therefore be controlled to take into account the total quantity of material being thawed. Given the urgency which often attends trauma care, the possibility of error in miscalculating the exposure time of a given quantity of frozen material is introduced. For example, if only a single bag of plasma is introduced into an oven set to operate for the time required to thaw two bags, overheating and damage to the material being thawed would result.
In the aforementioned U.S. Pat. No. 4,336,435, temperature probes associated with each rotary shaft are intended to take into account variations in the quantity of plasma being thawed by monitoring the temperature of each bag and controlling exposure time to prevent the plasma from being overheated. However, as previously suggested, this apparatus is not completely effective in preventing damage to the plasma during thawing, since the probe can measure the temperature at only a single point on the bag and cannot sense localized overheating in remote locations within the bag.
Thus, there is a need to provide a microwave apparatus for the rapid thawing of frozen plasma which avoids uneven heating of the material being thawed and which eliminates the possibility of error arising from variations in exposure time according to the volume of material being thawed.
Efforts have been made to devise an apparatus which overcomes the problems associated with cavity-type microwave ovens. In one such apparatus, a microwave illuminator comprising an upwardly-facing horn antenna has a microwave source disposed in its base. The horn disperses the microwaves across its cross-sectional area, thereby providing a electromagnetic field of approximately uniform intensity across the horn aperture. Additionally, the horn guides the microwaves in a substantially linear path, minimizing reflection of the microwaves and thereby eliminating the standing wave caused by superposition of incident and reflected waves. Thus, microwave "hot" and "cold" spots are minimized. The horn is filled with granular silica, or sand, a semi-solid material having a dielectric constant substantially equal to the dielectric constant of frozen plasma. The bag of frozen plasma is set in the mouth of the horn directly on top of the sand loading the horn. The granular silica permits an efficient coupling of the microwave energy into the bag of frozen plasma, since it minimizes dielectric discontinuities between the microwave source and the contents of the bag. Additional sand is then poured over the top of the bag of plasma to minimize the dielectric discontinuity along the surface of the bag distal to the microwave source. The sand loading the horn and surrounding the bag of plasma serves to couple the microwave energy to the frozen plasma efficiently.
This illuminator design suffers a number of disadvantages, however, First, to minimize the dielectric discontinuities between the bag of frozen plasma and the sand upon which it rests, special care must be taken when freezing the plasma to ensure that the bag is perfectly flat. Any container of plasma frozen in an irregularly shaped configuration will cause air spaces between the bag and the sand, creating a dielectric discontinuity which will reflect a portion of the incident wave and thereby contribute to nonuniform heating. Additionally, after continued use, the sand in the horn becomes packed down, changing the dielectric characteristics of the horn load and disrupting the power distribution. Further problems arise from the direct contact between the bag and the sand. The requirement of pouring sand over the bag is messy and can lead to irregular results. And, any condensation on the exterior of the cold bag could cause sand to stick to the bag ports so as to contaminate the plasma when it is later withdrawn. Finally, the sand surrounding the bag serves as a thermal insulator, preventing any localized heat build-up from dissipating.
Thus, there is a need to provide an electromagnetic illuminator which quickly, safely, and hygienically thaws frozen plasma.
There is also a need to provide an electromagnetic illuminator for thawing frozen plasma which accommodates irregularly-shaped bags.
There is yet another need to provide an electromagnetic illuminator which eliminates dielectric discontinuities but which does not thermally insulate the product being thawed so as to aggravate thermal runaway.